Generally, a key step in optical measurement technique is to make the probing beam focused onto the sample. Two methods are currently widely used. One method is to separate the last focusing lens from other components and only to adjust the focusing lens to focus the probing beam onto the sample. An example was illustrated in FIG. 1, the focusing is achieved by moving the last focusing lens up and down. The other method is to adjust the whole optical measurement system to focus the probing beam on the sample. For example, as shown in FIG. 2, the focusing is achieved by moving the whole optical system up and down (refer to the U.S. Pat. No. 5,747,813 and U.S. Pat. No. 5,486,701).
With the rapid development of semiconductor industry, it becomes very critical to apply optical technology to accurately measure the critical dimension (CD), spatial profile and material properties of three-dimensional structures formed by the single or multiple layer on wafers. While detecting 150 mm, 200 mm and 300 mm wafers, its surface may not be flat due to various reasons such as film stress. Therefore, auto-focusing for each measurement point is a key technique to achieve high accuracy and rapid measurement and ensure production of semiconductor production line across the whole wafer. And it is widely known that focusing the broadband probing beam into a small spot size on the sample surface is highly desired. The small spot size allows to measure the micro patterned structures and broadband spectrum is helpful for better measurement accuracy. There were some issues for the first approach in this case: lens usually has chromatic aberration which results in different wavelengths of light focusing on different locations and thus worsen the accuracy. It is also hard to find the lens materials with good transmission in the whole broadband wavelength range. The second approach inherits the same problem of lens aberration and has additional technical challenges. It is not a trivial job to adjust the entire optical system to focus and precise measurement is therefore difficult to be achieved.
A new approach was proposed as the above reasons, that is, focusing the broadband probe beam on the sample surface by using the curved mirrors (for example, refer to U.S. Pat. No. 5,608,526 and U.S. Pat. No. 7,505,133B1, U.S. Patent Application Publication No. 2007/0247624A1 and Chinese patent application Publication No. 101467306A). This method has advantages as below: the reflector mirror does not produce chromatic aberration in whole wavelength range, and has high reflectivity in wide wavelength range.
However, although the application of curved mirrors does not produce chromatic aberration and thus improve the focus and the accuracy, compared with lens, it is more difficult to align the optical path with curved mirrors. The adjustment of focal point and spatial orientation of curved mirrors was constrained by the incident light, often requiring the simultaneous adjustment of the entire optical system for better control of the output optical path and focusing point. For example, (1) elliptical mirror: While the spatial location of two focusing point is relatively fixed, the adjustable range of optical path and focusing position is very limited by adjusting the individual elliptical mirror after the incident light path was corrected. (2) Toroidal mirror: Although the two corresponding focusing points can be achieved in a certain range of incident angles, the spatial relationship of the two focusing points changes with the relationship between toroidal mirror and incident light. The correlations between two focusing points are complex and it is very difficult to achieve focusing. Another drawback is that its adjustable range is small and is easy to create image aberrations. (3) Off-axis parabolic mirrors: The adjustable range was very limited because the aberrations were resulted as the angle of off-axis parabolic mirrors changes relative to the direction of incident light. While a wide range of the focusing position can be achieved by moving the off-axis parabolic mirror along with the direction of the collimated light beam, the relative position of focusing point to the off-axis parabolic mirror center cannot be changed. This also limits the adjustable range of the focusing points. In summary, the use of a single curved mirror itself does not produce chromatic aberration, but it is difficult to control the direction of the optical path and focusing positions. Furthermore, the polarization of beam will be changed after reflected by a single mirror. Take an aluminum reflector mirror as an example, the reflection coefficient rs and rp of S and P polarized light were changed with the incident angle, the amplitude and phase difference between the S and P polarized light vary with the angle and the wavelength of incident beam. In short, because the polarization states S and P with the polarization direction orthogonal to each other have different reflectivity and phase change, after being reflected by a mirror, the polarization states of broadband beam varies, resulting the control of the change of beam polarization difficult (for example, refer to U.S. Pat. No. 6,829,049B1 and No. 6,667,805).
In addition, the beam polarization control capability of the spectrometer determines the scope of its applications. Take Optical Critical Dimension (OCD) equipment as an example. Such equipment is widely used in integrated circuit manufacturing lines for process controls. The OCD tools can measure the critical dimension (CD), three-dimensional profile of periodic pattern on sample surface, thickness and optical constants of multilayer film materials by collecting reflectance spectra and phase characteristics of the polarized beam from the sample surface and fitting numerical simulation results. For this kind of applications, the focusing system of spectrometer must be able to control the beam polarization in the process of focusing and optical signal collection.
Furthermore, when the spectrometer without polarizer was used to measure the sample with periodic structures, as mentioned in China patent application No. 201010270454.2, the incident beam must be natural light because the rotation angle of incident beam cannot be adjusted relative to the anisotropic angle of samples. In theory, the natural light emitted from light source is required to arrive on the sample surface either maintaining absolutely polarization or passing through none of polarization components. The anisotropic samples cannot be measured if polarization states were not maintained; under this circumstance, the measured values change as the anisotropic samples rotate. Therefore, the spectrometer capable of measuring the anisotropic samples while without polarization control components included demands the high quality of optical elements and the sophisticated adjustment of the optical path. The light reflected by the sample is partially polarized; starting from the light source to the detector, in theory, the polarization of the incident beam either was maintained completely or no polarization-sensitive component was present in the path. For example, if a polarization sensitive component was encountered in the path, a depolarizer is required, thus it will reduce the signal to noise ratio. Moreover, the above problem cannot be corrected by numerical methods.